When people ask what size air conditioner they need, they usually want a number they can take shopping. The honest answer is that the right size depends on the room — not just its floor area — and getting it wrong in either direction causes real problems. Here's how sizing works, the rough rules of thumb, and why a proper survey beats any online calculator.
What kW actually means
Air conditioners are rated by cooling output in kilowatts — how much heat they can pull out of a room, not how much electricity they draw. Domestic units come in a handful of standard sizes: roughly 2.0, 2.5, 3.5, 5 and 7kW. You'll sometimes see BTU instead, especially on portable units; 1kW is roughly 3,400 BTU, so a "12,000 BTU" unit is about 3.5kW.
Because a modern split system is a heat pump, the electricity it uses is far lower than its output — it typically moves three or more units of heat for every unit of electricity it consumes. That's why running costs are lower than most people expect. The same machine runs in reverse in winter and heats the room efficiently too — worth knowing, because a good surveyor will size with heating in mind if you plan to use it that way.
A rough rule of thumb
As a starting point — and only a starting point — UK rooms typically need somewhere around 100–150 watts of cooling per square metre of floor. That puts most homes into one of the standard sizes:
| Room | Typical capacity |
|---|---|
| Small bedroom or home office (up to ~15m²) | 2.0–2.5kW |
| Average living room or main bedroom (15–25m²) | 2.5–3.5kW |
| Large living room or open-plan space (25–40m²) | 3.5–5kW |
| Very large open-plan area (40m²+) | 5–7kW, or two indoor units |
Treat that as a sanity check for quotes, not a specification. Two rooms with identical floor areas can genuinely need different sizes, and here's why.
What pushes the number up
- Glazing and aspect. Big windows facing south or west are the single largest source of summer heat gain. A room that catches full afternoon sun can need a size up from an identical room facing north.
- Loft rooms and conversions. Heat comes through the roof as well as the windows, and warm air from the rest of the house rises into them. Lofts routinely need more capacity than their floor area suggests — often the reason people want air conditioning in the first place.
- Ceiling height. The rule of thumb assumes ordinary ceilings. Tall Victorian rooms and double-height spaces hold more air to cool.
- People and appliances. A home office full of computers, or a kitchen-diner with the oven on, adds heat the unit has to remove.
- Insulation. A poorly insulated room lets heat in during summer just as it leaks heat out in winter.
Why undersizing is a problem
An undersized unit runs flat out and still never reaches the temperature you've set — precisely on the hottest days, which are the days you bought it for. Running at maximum constantly also means more noise, more electricity for the comfort you actually get, and more wear on the compressor. It's a machine spending its whole life at a sprint.
Why oversizing is a problem too
The instinct is to go a size up to be safe. Resist it. An oversized unit hits the target temperature quickly, switches off, then switches back on a few minutes later — what installers call short-cycling. That stop-start pattern wears components faster than steady running does. It also ruins dehumidification: a unit only dries the air properly during longer run cycles, so an oversized one leaves the room cool but clammy — more cellar than fresh air. Modern inverter units soften this by throttling down rather than switching off, but they can only throttle so far; a unit that's badly oversized for the room still cycles. And you've paid more up front for the privilege.
What a heat-load calculation is
This is why a proper survey matters. A heat-load (or heat-gain) calculation is not mysterious: the surveyor measures the room, notes the glazing area and which way it faces, checks ceiling height and insulation, and asks how the room is used — how many people, what equipment, day or night. From that they work out roughly how much heat the room gains on a hot day, then pick the unit size comfortably above that figure. It takes a surveyor a few minutes with a laser measure.
The useful test for any quote: if nobody measured the room or asked about the windows, the size on the quote is a guess. And if you're thinking about more than one room, sizing interacts with system choice too — multi-split versus separate single splits is worth reading before you commit.
Getting it sized properly
One thing worth being clear on: whatever size you land on, fitting a split system is not a DIY job. The refrigerant lines mean installation must be done by an F-Gas certified engineer — that's a legal requirement in the UK, not a recommendation. There's no harm in measuring your rooms and checking quotes against the table above, but leave the pipework, refrigerant and electrical connections to the professionals.
Sizing is one of the few decisions you can't easily undo — the unit that's fitted is the unit you live with. Prices don't swing wildly between adjacent sizes (here's what installation typically costs), so there's no reason to accept a guess. Tell us about your home and we'll match you with a vetted local installer who'll do the heat-load calculation as part of a no-obligation survey — start with a quick estimate here.